/**

* Question Link: https://leetcode.com/problems/reverse-string/

* Primary idea: Two pointers, iterate the string and swap two characters each time

*

* Note: Swift does not have a way to access a character in a string with O(1),

* thus we have to first transfer the string to a character array

* Time Complexity: O(n), Space Complexity: O(n)

*

*/

class ReverseString {

func reverseString(s: String) -> String {

var left = 0

var right = s.characters.count - 1

var chars = [Character](s.characters)

while left < right {

_swap(&chars, left, right)

left += 1

right -= 1

}

return String(chars)

}

private func _swap(inout chars:[Character], _ p: Int, _ q: Int) {

var temp = chars[p]

chars[p] = chars[q]

chars[q] = temp

}